1. Field of the Invention
The present invention relates generally to a display system, such as a liquid crystal display system. The present invention also relates to a system for providing electrical driving of a common electrode which is on an unpixellated substrate of a display system. More particularly, the invention relates to a system for electrically driving the common electrode of a display system to various voltages in a controlled phase relationship to the update of pixel data.
2. Background of the Related Art
A class of display systems operate by electrically addressing a thin, intervening layer of electro-optic material, such as liquid crystal, which is positioned between two substrates. In these display systems, it is important to achieve good display characteristics including: color purity, high contrast, high brightness, and a fast response.
High independence of frames or subframes ensures the lack of coupling between intensity values at a given pixel from one frame to the next. For example, if a pixel is to be at its brightest grey level during a first frame and then at its darkest grey level at the next frame, then a high independence would ensure that this is possible whereas a low independence would cause to pixel to appear brighter than the darkest grey level during the second frame. This coupling can cause problems such as motion smearing. High frame-to-frame independence is important whether or not the display is a color or black-and-white display.
The level of contrast achievable is determined by the range of intensity attainable between the brightest grey level and the darkest grey level for a given pixel within a given frame or subframe.
In addition to contrast, it is desirable that the display be capable of displaying a bright image since the brighter image can be viewed without the necessity of external light sources or strong ambient light.
Finally, the speed of display is determined its ability to display one frame after the other at a high rate. If visual motion is to be displayed, flicker and other problems can be avoided only if the full color frames are displayed at a rate of least 30 Hz.
This speed requirement becomes even more stringent if the display does not contain a red, green, and blue pixel all at one pixel location but instead only has a single pixel. One type of such a display is a color sequential liquid crystal display as discussed in "Color-Sequential Crystalline-Silicon LCLV based Projector for Consumer HDTV" by Sayyah, Forber, and Efrom in SID digest (1995) pages 520-523. In those type of displays, if a display requires the sequential display of the red, green, and blue subframes, those subframes must be displayed at yet a higher rate than 30 Hz and preferably greater than 90 Hz to avoid flicker. For color sequential displays, high frame or subframe independence is required to display images with good color purity.
Any of the general display systems that operate by electrically addressing a thin, intervening layer of electro-optic material, such as liquid crystal, which is positioned between two substrates include the following characteristics. At least one of the two substrates is transparent or translucent to light and one of the substrates includes a plurality of pixel electrodes. Each pixel electrode corresponds to one pixel of the display, and each of the former may be driven independently to certain voltages so as to control the intervening electro-optic layer in such a way as to cause an image to be displayed on the electro-optic layer of the display. Sometimes each pixel can include color triad of pixel electrodes. The second substrate of such a prior art display system has a single electrode, known as the common electrode, which serves to provide a reference voltage so that the pixel electrodes can develop an electric field across the intervening layer of electro-optic material.
One example of such a system is a color thin film transistor (TFT) liquid crystal display. These displays are used in many notebook-sized portable computers. Colors are generated in these displays by using RGB pixel triads in which each pixel of the triad controls the amount of light passing through its corresponding red, green, or blue color filter. These color filters are one of the most costly components of a TFT display.
The major obstacle of display systems of this type is that the results of replicating the pixel electrodes, data wire, and thin film transistors, three times at each color pixel are increased cost and reduced light transmission, requiring more peripheral backlights and increased power consumption.
The other issues of high frame-to-frame independence, high contrast, and brightness become even more difficult to achieve as display rates increase.
Many approaches have been implemented to improve display characteristics of the above type displays. One common approach involves the use of a common electrode driving circuit and driving that common electrode with as flat a common electrode rectangular driving voltage as possible. By doing so, the voltage across the liquid crystal portion at that pixel is more constant, which in turn should yield improved contrast and pixel brightness.
For example, U.S. Pat. No. 5,537,129 discloses a display system with a common electrode which attempts to achieve a flat rectangular common electrode driving voltage. Referring to FIG. 2 of that patent, a common electrode 24 is connected to its driving circuit 20 through a resistor 3b. This corrects for resistive losses at 3a and capacitive coupling to the common electrode 24 from pixels and data wires. This ensures that detection device 21 with a high input impedance can be used to make a correction so the output voltage appears to be more rectangular-like. FIGS. 5, 9b, 11(c), and 11(d) of that reference all show the desired rectangular waveforms.
Another example of this is shown with U.S. Pat. No. 5,561,442. Referring to Figure which shows that with the properly applied common electrode voltage Vc(t) when coordinated with the previous gate wire voltage Vs(t) and the current gate wire voltage Vg(t), can yield a flat rectangular voltage V(t)-Vc(t) across the liquid crystal (C.sub.LC). This scheme involves a complicated modulation scheme coordinating modulation voltages at gate wires in relation to the modulation of the voltage at the common electrode in order to achieve their desired flat rectangular modulation of voltage across the liquid crystal.